1. Field of the Invention
The present invention relates to methods for manufacturing powdered raw materials for dielectric ceramics which are effectively used for ceramic capacitors.
2. Description of the Related Art
In manufacturing ceramic capacitors, various metal elements are generally added to dielectric ceramic compositions such as BaTiO3, SrTiO3 and (Ba,Ca)(Ti,Zr)O3 to improve the electric characteristics and sintering properties thereof. A method currently employed for adding metal elements is, for example, to coat compounds containing metal elements on the surfaces of the compositions for the dielectric ceramics. Various coating methods have been proposed. The followings are examples:
(1) Carbonate salts or oxides of metal elements, or mixtures of the two, are added to a powdered dielectric ceramic composition, and this is then blended while being pulverized mechanically using a ball mill, a sand mill, or the like.
(2) A powdered dielectric ceramic composition is dispersed in an organic solvent dissolving compounds containing metal elements to form a slurry, and the organic solvent is then removed by evaporation.
(3) A powdered dielectric ceramic composition is dispersed in an aqueous solution dissolving metal salts containing metal elements to form a slurry. Then, a precipitate from the aqueous solution is separated by filtration or the aqueous solvent in the slurry is removed by evaporation (for example, as described in Japanese Laid-open Patent Application No. Sho-63-141204).
However, the metal elements in method (1) are not uniformly dispersed in the powdered dielectric ceramic composition. Hence, for example, when the metal element functions as a sintering aid agent, desired sintering characteristics cannot be achieved unless an excess amount of the sintering agent is added. Consequently, the relative dielectric constant may be decreased and a sintered product having extremely low insulating resistance in some portions thereof may be produced due to locally inconsistent concentrations of the metal elements.
In method (2), the compounds containing the elements uniformly coat the surfaces of the powdered dielectric ceramic composition so that the elements are uniformly dispersed in the basic powdered dielectric ceramic composition. Consequently, powdered raw materials for the dielectric ceramics having superior sintering characteristics, electric characteristics, and the like can be obtained. However, large quantities of organic solvents must be used.
In method (3), there are problems in that when precipitation occurred, uniform precipitation throughout the entire slurry was difficult to obtain and localized precipitation proceeded, in some cases, depending on reaction conditions. When this occurred, the elements were not uniformly dispersed in the basic powdered dielectric ceramic composition. In addition, when the aqueous solution was evaporated and dehydrated, anions in the aqueous solution dissolving the metal salts sometimes remained as impurities in the basic powdered dielectric ceramic composition.
Accordingly, one object of the present invention is to provide an inexpensive method for manufacturing a powdered raw material for a dielectric ceramic by using an aqueous solvent. In this method, elements are uniformly dispersed in a powdered dielectric ceramic composition while contamination by impurities is suppressed.
The present invention provides a method for manufacturing a powdered raw material having compounds containing metal elements on the particle surfaces thereof, comprises the steps of dispersing a powdered dielectric ceramic composition in water to form a slurry, adding a silicon compound to the slurry to coat the powdered raw material of a basic powdered dielectric ceramic composition, adding a solution containing a metal element constituting the compound and a precipitating agent reactive with the metal elements to form a precipitate, while the slurry is being stirred, and rinsing and drying the slurry.
Another method for manufacturing a powdered raw material having compounds containing metal elements on the particle surfaces thereof comprises the steps of dispersing a powdered dielectric ceramic composition in water to form a slurry, adding a pH buffer solution, a solution containing a metal element constituting the compound and a precipitating agent reactive with the metal elements to the slurry to form a precipitate while the slurry is being stirred (the addition of the pH buffer solution is performed prior to the additions of the solution containing the metal compounds and the precipitating agent), and rinsing and drying the slurry.
Still another method for manufacturing a powdered raw material for a dielectric ceramic having compounds containing metal elements on the particle surfaces thereof comprises the steps of dispersing a powdered dielectric ceramic composition in water to form a slurry, adding a silicon compound to the slurry to coat the powdered raw material of a basic powdered dielectric ceramic composition, adding a pH buffer solution, a solution containing a metal element constituting the compound and a precipitating agent reacting with the metal element to the slurry to form a precipitate while the slurry is being stirred (the addition of the pH buffer solution is performed prior to the additions of the solution containing the metal elements and the precipitating agent), and rinsing and drying the slurry.
The silicon compounds may comprise either a silica sol or a silicate salt.
The amount of the precipitating agent to be added may exceed the stoichiometric equivalent for the precipitation.
The basic powdered dielectric ceramic composition may comprise BaTiO3.
The metal element contained in the solution is selected from at least one of Dy, Mg and Mn.
The precipitating agent may comprise an aqueous NaOH solution.